EXTRACTION OF METALS
EXTRACTION OF METALS
Definition: Extraction of metals refers to the process of removing or obtaining metallic elements from their respective ores
What is an ores?
An ores is the mineral from which particular metal can be extracted conveniently and economically. example Haematite (Fe2O3), Iron pyrite (FeS2), galena (PbS), Zinc blende (ZnS), tin- stone or cassiterite (SnO2)
What are minerals?
- Are naturally occurring metallic compounds found in earth’s crust which can be obtained by mining, example Magnetite (Fe3O4), Gypsum (CaSO2.2H2O), Bauxite ( Al2O3.2H2O), Malachite (CuCO3.Cu(OH)2), Limestone (CaCO3) etc.
NB; All minerals are not ores but all ores are minerals
NATURAL OCCURRENCE OF METAL ORES
Metals occur naturally in two states
I. NATIVE OR UNCOMBINED STATE
II. COMBINED STATE
A. NATIVE STATE
The element are said to occur in the native state when they are found in their elementary form (free metals)
Example; Copper (Cu), Platinum (Pt), Gold (Au), Silver (Ag) and mercury (Hg)
B. COMBINED STATE
The elements are said to occur in combined state when they are found in the form of the compounds . Generally the reactive metals occur as oxides, sulphides, sulphates, silicates, carbonates, chloride, nitrates etc .The element that occur in combined state include Fe, Cu, Al, Pb, Sn, Ca, Mg, Na, Mn, Cr, Co etc
QUESTION; what are main ores from which tin, copper and aluminum are extracted?
NATURAL OCCURRENCE OF METAL ORES OF TIN, COPPER, AND ALUMINIUM
Aluminium is the third element in abundance after silicon and oxygen. It is the most abundant metal in the earth’s crust. Aluminium is a reactive metal and hence does not occur as a free metal.
The following are different forms in which aluminium occur in nature;
I. Corundum(Al2O3) -Free oxide
II. Bauxite (AL3O3.2H2O) -Hydrated oxide
III. Silicates (Al2Si2O7.2H2O), KAlSi3O8, KH2Al3(SiO4)3
Kaolin (China clay) Feldspar Mica
IV. Cryolite (Na3AlF6)
Aluminium ores have different colours. The colours are due to the presence of other metals which are impurities, example Ruby (red) containing Iron and Titanium, Sapphire (Blue) containing cobalt and Titanium
Bauxite is the most economic ore from which Aluminium is extracted. Deposits of bauxite are exploited in South–East Europe, India, Australia, Brazil, USA, and West Indies
b) TIN (Sn)
Tin does not occur as free element because it is moderately reactive metal .Tin occur in form of oxide (tin-stone SnO2) and sulphide (tin –pyrite CuS.FeS.SnS2). The important ore of tin is cassiterite (tin- stone) this one contain (0.5-10)% of the metal as SnO2 and the rest are impurities i.e sulphides of Fe, Cu and lungstates of sand, Silicious matter, earth matter etc.
The miners call tin-stone as black tin to distinguish it from the white tin which has the name given to metal. Tin deposit occur in Malaysia, Indonesia, Bolivia, China, Burma, Thailand and Nigeria
c) COPPER (Cu)
Copper occur as free element as well as in combined state. In combined state copper occur as copper pyrite (CuFeS2), azurite (2CuCO3.Cu(OH)2) and malachite (CuCO3.Cu(OH)2). Malachite and azurite are basic carbonates. Deposit of copper occurs in Zambia, Democratic Republic of Congo. CuFeS2 is the chief ore of copper
NB; i) Copper occur as free metal in late superior in Canada
ii) Copper also occur naturally in form of oxide (ruby ore – Cu2O)
iii) Minerals and ores differ from one another due to the fact that minerals contain a low percentage of the metal while the ores contain a large percentage of the metal
iv) The metal cannot be extracted from the mineral; it can be extracted from an ore.
STAGES OF METAL EXTRACTION
There are three major stages of metal extraction
I. Concentration of ore
III. Refining of purification
I. CONCENTRATION OF ORE
Concentration of ore means increasing the metallic content of the ore by removing the impurities (gangue). The ore which is mined usually contains large amount of rocky impurities such as sand, clay, limestone which are called gangue.
Definition; Gangue is the worthless or useless earthy impurities which are found in an ore.
Before concentration proper separation of ore is carried out, the ore is broken up into lumps which are then ground down to a fine powder which is suitable for the next operation. Concentration of the ore is carried out by any of the following methods depending upon the nature of the ore and gangue.
1) GRAVITY SEPARATION (WASHING WITH WATER)
This process is applied where there is well marked difference in the densities of the gangue and the ore. The crushed ore is washed by current water on a slopping table filled with a series of corrugated boards. The table is kept vibrating all the time. The lighter particles of gangue are washed off leaving behind heavier particles of the ore.
2) MAGNETIC SEPARATION
The magnetic impurities present in an ore are separated by a magnet .The powdered ore allowed to fall on a rubber belt moving around two rollers. One of the roller is a strong magnet, the ore moves with the belt towards the magnetic roller and then the belts take a turn near the magnet, the magnetic components is attracted by the magnet and form a separate heap. The non–magnetic components (e.g. tin-stone) is separated from magnetic component (e.g. Wolframite FeWO4).
3) FROTH FLOTATION
This method is commonly used for concentration of the low sulphide ores like galena (PbS), copper iron pyrite (CuFeS2 or Cu2.Fe2S), Zinc blende (ZnS). This method of concentration is based on the different wetting characteristics of the sulphide ore and the gangue particles.
The finely powdered ore is mixed with water in a tank (floatation all). Some amount of pine oil (froth) and sodium ethyl xanthates collectors are added to it. Air is then blown through the mixture vigorous. The sulphide ore particles which are wetted by the oil rise up to the surface of liquid. The sulphide ore from stable froth which is skimmed off. The gangue particles which are wetted by water sink to the bottom of the tank (floatation cell). The froth with sulphide particles are collected in a separate tank
NB; The role of sodium ethyl xanthate is to collect sulphide ore particles. This compound is attached to sulphide ore particles and makes them water repellant. As a result the sulphide ore particles pass on into froth easily.
In this method the ore containing impurities is leached with aqueous solution of a suitable dissolving reagent so that the metal in an ore is converted into simple salt or complex compound while the impurities remain insoluble in the reagent which ore removed by filtration. The metal is then extracted from the simple salt or complex compound formed example impurities in bauxite (Al2O3.2H2O) i.e FeO, Fe2O3 and SiO2can be removed by heating. The crushed bauxite with sodium hydroxide solution (Bayer’s process). Aluminium present in the ore is converted into a soluble complex compound (aluminate complex –NaAlO2) while the impurities (i.e FeO, Fe2O3 and SiO2) remain insoluble and hence are filtered out.
5) CALCINATION OF THE ORE
Calcination of ore refers to the process of heating the concentrated ore strongly in a limited supply of air or in the absence of air. The process of
Calcination brings about the following changes;
a) The carbonates ores are decomposed to their representative metal oxide
b) Water of crystallization in the hydrated oxide gets lost in form of water vapour.
c) Organic matter, if present in the ore gets lost or expelled and the ore becomes porous
Roasting is the process of heating the concentrated ore strongly in excess air or oxygen below its melting point. Roasting can be done at moderate or high temperature. The roasting is generally for sulphide ores, when roasting takes place at moderate temperature some portion of the sulphide ore like galena (PbS), Zinc blende (ZnS) is converted into metallic oxide and remaining portion is converted into metallic sulphates.
When roasting takes place at high temperature in the presence of oxygen, some sulphide ore give metallic oxide.
The sulphide ores of some metals like Cu, Pb, Hg, Sb etc when heated strongly in excess of air or oxygen are reduced directly to the metallic elements
THE DIFFERENCE BETWEEN CALCINATION AND ROASTING
1. Calcination is used for hydrated oxides or carbonates while roasting is for sulphide ores.
2. In calcination the ore is heated strongly in the absence of air or oxygen while in roasting the ore is heated in excess air or oxygen.
3. In calcination the hydrated oxide or carbonate is converted metallic oxide while roasting the sulphides ore is converted into metallic oxide or sulphates or metallic element.
During the extraction of a metal from its ore, the metal ions are reduced to metal atoms by accepting electrons. Thus extraction of a metal from its ore is a reduction process. Reduction of a metal ores by thermal or electrolytic reduction method
In thermal reduction method heat is used in extracting the metal from its ore. In this method a suitable reducing agent may be used example coke, coal, Al, Co, Carbon, Mg, Ca etc
The elements which are extracted by this method are those which are moderately reactive example Zn ,Pb ,Fe , Sn, Cu etc Reaction takes place either in an electric furnace or blast furnace or suitable container
In electrolytic reduction, electricity is used in extracting the metals from its ore. This method is used in extracting metals which are strongly electropositive (more reactive) like Na, K, Ca, Mg and Al. These metals cannot be reduced by the normal reducing agent like Co or Carbon because their compound (ores) is stable. Even if the metal is formed, carbon reacts with the metal produced to form carbide at high temperature. These reactive metals are extracted by passing through their molten or fused chlorides oxides or hydroxides.
II. PURIFICATION (REFINING) OF IMPURE METALS
Purification is a process of removing impurities from the extracted metal. The metal obtained contain many number of impurities and hence need to be purified .The impurities include other metals, non-metals, unreduced oxides and sulphides of metal slag. The method to be used in purification of a particular metal depends on the nature of the metal to be purified and the impurities to be removed and the purposes for which the metal is to be used
The method used for refining the impure metals can be categorized in four categories
1. PHYSICAL METHODS
a) LIQUAFACTION PROCESS
The process is used for the purification of the metal which melts at lower temperature than each of the impurities associated with it. This method is for purification of Sn, Zn and Pb.
b) FRACTIONAL DISTILLATION
This process is used to purify those metals which are volatile and the impurities in them are non-volatile and vice versa. The vapour of the metal is condensed in a separate vessel while the non-volatile impurities remain behind in the vessel in solid state. Example Purification of Zn where As, Cd, Pb and Fe are impurities.
2. CHEMICAL METHOD
a) OXIDATIVE PROCESS
This process is used to remove metallic and non-metallic elements present as impurities example Mn, Cu, Pb, Sn, Fe, Ag, C, P, S, Si etc. When the oxygen or air is passed through the impure molten metal, the impurities are easily oxidized into their oxides
i) May form scum on the surface of the metal and hence can easily be removed by skimming
ii) May be volatile and hence they can escape through the mouth of the furnace
iii) May form a slag with the lining on their side surface of the furnace and may thus be removed
b) THERMAL DECOMPOSITION METHOD
In this method, the impure metal is converted into a suitable volatile compound which on being heated decomposes to give pure metal, example purification of nickel by Mond’s process (carbonyl process)
This process is used for extraction of silver and gold. Both silver and gold form amalgam with mercury (example Au/Hg and Ag/Hg). When Au /Hg or Ag/Hg is distilled in an irot retort, mercury being more volatile, distilled off and Au or Ag metal is left behind in the retort.
Purification by electrolytic method given metal of high purity. In this process, the impure metal is made the anode of the electrolytic cell while the pure metal is made the cathode of the cell. An aqueous solution of suitable sample or complex salt of the metal having some corresponding acid (if necessary ) is used as an electrolyte .When the electric current of appropriate strength is passed through the electrolyte, the metal from the impure plate (anode) migrates to the pure plates (cathode) where it is deposited. The soluble impurities go into the solution while the insoluble settle down at the bottom, below the anode. The matter settled below the anode is called anode mud or anode sludge. Sometimes the anode mud contains valuable metals which can be extracted from it .The metal that can be purified by this method include Sn, Pb, Cu, Ag, Ni, Zn, Cr etc
THERMAL REDUCTION OF TIN STONES (SnO2)
Tin is generally extracted from tin stone (SnO2). This contains SiO2 as an impurity (acidic impurity). In order to remove this impurity and reduce tin stone to tin metal, the oxide is mixed with anthracite coal and a basic flux like lime (CaO). The mixture is heated in a reverboratory furnace at 12000C-13090C. Then tin stone (SnO2) is reduced to tin metal (molten tin) and the impurity SiO2 combine with CaO to form a slag of CaSiO3 (Calcium Silicate ). The molten tin (heavier) form the lower layer and slag form the upper layer.
The molten tin is tapped and cast into blocks or ingots. The block contains 70% of metallic tin
Some SnO is also formed during the reduction of SnO2 to Sn
The SnO and CaO react with silica (SiO2) to form slag of SnSiO3and CaSiO3
The slag of SnSiO3 and CaSiO3 being lighter floats on the molten tin metal as an upper layer which is easily removed. Thus SiO2 (Impurity) is removed inform of a slag. The tin metal can also be recovered from SnSiO3 by melting it with limestone (CaCO3)in a blast furnace or reverboratory furnace.
I. Flux is a substance which reacts with the gangue (impurities) during smelting to form fusible compound which can easily be removed from moltenmetal, example CaO and SiO2. The CaO is basic flux while SiO2 is an acidic flux.
The acidic flux removes the basic impurities example CaO, FeO, MgO etc. The basic flux removes acidic impurities like SiO2, P3O5 etc
II. Slag is a fusible compound obtained by the combination of the flux and gangue present in the ore.
i.e Flux + Gangue → Slag ⇒ CaO + SiO2 → CaSiO3
III. The gangue may be acidic (example SiO2) or basic (example Cao, MgO, FeO etc).
IV. Smelting is the process of conversion of roasted or calcined ore into molten metal using a suitable reducing agent at high temperature in a current of air.
ELECTROLYTIC REDUCTION (ELECTRO METALLURGY)
This method is used for extraction of metals which are very reactive and form very stable compounds for example the oxides, chloride, hydroxides etc of more reactive metal like Na, K, Mg, Ca, Al etc are very stable and hence cannot be reduced to free metals either by carbon or aluminium. However the oxides of the above metal can be reduced by carbon at very high temperature, but as soon as the metal is formed reacts with carbon to form metal carbide. Therefore the metals are obtained by reducing their suitable molten (fused) salts like chlorides, hydroxides etc by electrolytic reduction. The metal is liberated at the cathode. An aqueous electrolyte is not used in electrolytic reduction because the metal obtained at the cathode reacts with water present in the solution
In order to make an electrolytic reduction successful the following condition (criteria) must be satisfied;
a) The liberated metal should not be miscible with the melt (soluble in the melt) for example potassium cannot be obtained by electrolysis of (KCl and CaCl2) melt, since the liquid potassium is soluble in CaCl2
b) The products obtained as a result of electrolysis may react and hence must be collected separately
c) Some other salts may be added to lower the Melting point of compound that melt at very high temperature. The decrease in Melting point makes the process of electrolysis take place at the lower temperature.
d) The cell and electrodes should be such of material which are not affected by the electrolyte or the products obtained after electrolysis
ELECTROLYTIC REDUCTION OF ALUMINIUM FROM ALUMINIUM OXIDES(Al2O3)
In the electrolytic reduction of aluminum pure Al2O3 is mixed with Cryolite (Na3AlF6) and then the mixture is melted in an electric furnace. The fused mixture of Al2O3 and Na3AlF6 is taken in an iron tank lined with carbon from inside. This carbon lining acts as cathode and carbon rods which are dipped into the molten Al2O3 and Na3AlF6 acts as anode. On passing electric current aluminium metal in the molten state is liberated at the cathode and sinks to the bottom of the cell from which is removed periodically (time to time).
Oxygen from the electrolyte is liberated at the anode. Some of the oxygen combines with the carbon anodes to form carbon monoxide and carbon dioxide. The anodes therefore are gradually worn out.
NOTE; The cryolite serves as a solvent and not decomposed
Reaction at the cathode (reduction)
Reaction at the anodes (oxidation)
Thus O2, CO and CO2 are liberated at the anodes
PURIFICATION OF BLISTER COPPER
Copper obtained from copper-Iron pyrite (CuFeS2) is called BLISTER COPPER. This copper contained many impurities like Fe, Ni, Zn, Ag, Au etc.
In refining blisters copper, a thick plate of impure copper is made the anode and a thin plate of pure copper is made the cathode. The cathode is coated with graphite and oil which helps in scrapping the copper easily that gets deposited on the cathode 15% CuSO4 solution containing 5% of H2SO4 is used as an electrolyte.
On passing electric current through the electrolyte ionization of CuSO4 tends to take place
The copper ions (Cu2+) obtained move towards the cathode where they accept electrons and become neutral Cu atoms. Thus cathode (pure copper plate) goes on becoming thicker and thicker in size as the electrolysis goes on.
The ions move towards the anode where they lose electrons and become sulphate radicals. These radicals have strong tendency to combine with Cu atoms of copper anode (impure copper plate) to form CuSO4 which move into the solution which they are again ionized to give Cu2+ ions. These Cu2+ ions migrate to the cathode where they are deposited as free Cu atoms.
On adding equation (i), (ii) and (iii) we obtain;
Or Cu (impure copper) → Cu2+ + 2e– (oxidation)
The equation above represents the anode reaction (oxidation). This reaction indicates that the anode goes on becoming thinner and thinner in size as the reaction proceeds. From the reaction at the (anode), the pure copper is transferred from impure copper plate to pure plates (cathode).
The soluble impurities like Fe2+, Ni2+, and Zn2+ pass into solution as sulphates while the insoluble impurities (example Ag and Au) which are not affected by H2SO4-CuSO4 solution settle down below the anode as ANODE MUD or ANODE SLUDGE.
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